1. Technical Field
This invention relates to the art of draw forming metals, and more particularly to determining the change in shape of a metal sheet during binder closure and before draw punch impact.
2. Discussion of the Prior Art
In a draw forming process of sheet metal there are two stages: (1) the binder wrap (preforming stage), and (2) the punch and die contact stage. The sheet blank is gripped peripherally by the binder ring, which ring may have large curvatures deviating from a flat plane along two or more edges. In stage one, the sheet is laid on the lower binder surface of a die and the upper binder ring comes down to set the binder shape, called the binder wrap. In stage two, the punch travels down through the upper binder ring to form a contoured panel shape. In the punch stage, the sheet is drawn between the binder ring and die and feeds into the interior shape to accommodate deep draw of the latter.
It is necessary to compute the binder wrap configuration of a sheet to be able to determine, with further analysis, the punch and die closure in the second stage. The interior, ungripped portion of the sheet blank is virtually suspended; its deformed shape will be complex as a result of the weight of the sheet and as a result of the forced peripheral curvatures.
Sheet metal draw blanks which are not initially contacted by the draw punch in a centralized location of the suspended portion of the blank are likely to form wrinkles in the blank when fully drawn. In order to design the punch to contact the blank in such a centralized location for any particular application, the deformed shape of the blank must be known or determined as it is gripped in the binder wrap stage to permit punch/die redesign.
The prior art has heretofore used essentially three teachings: (i) a trial and error method of making binders and punches/dies, followed by rework and redesign until the desired shape is obtained; (ii) a geometric method based on fitted surface projections of the closest geometrical shape at each segment of the binder ring shape; and (iii) prediction of the binder wrap using a linear theory where the sheet deflection is assumed proportional to the applied load (a standard finite element program, such as NASTRAN, is commercially available to carry out the linear theory). The first method is too expensive and time consuming, the second method is too simplistic, leading to poor quality of draws because the mechanical properties, thickness and friction characteristics of the metal are not considered, thus requiring costly tryouts to compensate for inaccuracies. The last method is classic linear theory and is not valid to compute large deflections. An article representing the use of the last method is Takahashi et al, "Evaluation Methods of Press Forming Severity in CAD Applications", Computer Modeling of Sheet Forming Process, Edit by Nang C. Tang, The Metallurgical Soc , pp. 37-50, 1985.